The present invention relates to target location, and more particularly, to a passive method of locating targets.
In electronic warfare, the enemy typically employs high-power radars, navigation beacons and identification friend or foe (IFF) equipment to detect penetrating aircraft and to aid and recognize its own aircraft. When the radar detects a penetrating aircraft, a radar operator interrogates it with an IFF signal to determine its identity. If the aircraft is friendly, it responds to the IFF interrogation with a transponded signal, usually on a different frequency than that of the interrogation. During this short time of response to interrogation, the friendly aircraft is an active emitter. The command and response sequence can be accurately timed to determine the range between the interrogator and the aircraft. If an unfriendly aircraft is detected, the radar operator vectors a fighter aircraft to intercept the penetrating aircraft.
In the setting described above, it is very desirable that the penetrating aircraft passively locate the enemy radar and interceptors in range and in azimuth without using the active sensors. To this end, the penetrating aircraft carries direction finding equipment such as an electronic support measure (ESM) system, for example. However, the direction finding equipment may be any type of SONAR or electromagnetic system which includes detection equipment suitable for detecting target emissions. The detection equipment may be located on any type of vehicle, including an aircraft, submarine or surface ship. Consequently, due to the generic applicability of the present invention, the vehicle carrying the detection system is hereinafter referred to as a platform.
Conventional electronic ranging and geolocation techniques do not fully utilize available information, and rely on signals originating from active search radars or bistatic returns provided by echoes from other targets in the vicinity. The techniques fall into roughly three categories.
The first category includes those that rely on long baselines and use either crossing of bearings or hyperbolic lines of position derived from time difference of arrival. These techniques require either multiple platforms or special tactics. An example of multiple platforms is a system with two receivers located in each wing tip of an aircraft, for example. This approach acquires theoretical information with which to compute the range to the emitter. An example of special tactics requires that a single platform fly along a baseline while taking measurements over a long time. Multiple platforms result in a more costly and complex system. Flying along a baseline requires that the emitter be stationary and preferably off to the side of the platform thus rendering the system less effective for emitters directly ahead of the platform. Furthermore, the process requires a relatively long time, and is inconsistent with adequate reactive maneuvers by the platform while under enemy attack. These disadvantages make it impractical for single platform penetration missions.
The second category includes techniques that employ bistatic ranging with prior knowledge of one or more of the sides of the triangle formed by the emitters and a receiver. This information, however, may not be available or current immediately prior to a mission. Likewise, it may not be sufficiently accurate in a dynamic engagement mission. This work is described in U.S. Pat. No. 4,370,656 to Frazer and Lewis.
The third category includes techniques that employ bistatic ranging along with determination of one or more of the angles of the triangle formed by the platform, an emitter and a secondary reflector. One such technique determines the angle between the range vectors from the platform to the emitter and from the platform to a reflecting target by directional measurements relative to the platform. A second approach determines the angle between the range vectors from the emitter to the platform and from the emitter to a secondary target. This is done by measuring the time delay as the main radar beam sweeps through the platform and the secondary target. The measurement is taken as the time delay between the passing of the main beam and the measurement of the main beam reflection from the secondary target. This technique requires knowledge of the scan time of a search radar. This technique is described in U.S. Pat. No. 4,670,757 to Munich and Schecker. Either technique has limited areas of application since modern radars may not scan in a regular pattern or with a constant scan speed.
Accordingly, it is an objective of the present invention to provide a method of range and azimuth determination that utilizes all available active emissions from other sources. Another objective of this invention is to provide a method for measuring range and heading information that utilizes only momentary emission from secondary sources such as enemy IFF transmissions. It is a further objective of the present invention to provide a method for measuring range and heading which utilizes the responsive nature of the IFF transmissions and utilizes known internal delays of IFF transmissions responsive to radar interrogation. Yet another objective of this invention is to provide a method for measuring range and heading which utilizes existing, fielded and operational, direction finding equipment and systems. A still further objective of the present invention is the provision of a method for measuring range and heading which operates both in a bistatic mode or transponded mode, depending on available signals to be exploited. Another objective of this invention is to provide a method for measuring range and heading which obtain solutions of target locations that is substantially instantaneous in time.